In the film, Jon adopts Odie from his vet Liz's animal hospital. In the comic strip, Odie was originally the pet of Jon's friend Lyman.
In the film, Nermal is one of the cats who lives on the cul-de-sac. In the comic strip, Nermal - a kitten - originally belonged to Jon's parents.
The plot to the movie is based on the book and TV special from the early-'80s, "Here Comes Garfield". In it, Odie gets abducted by the dog catcher, and Garfield comes to his rescue.
Joe Towne who plays a technician in the film (as Ariel Joseph Towne) was hired during production as the on-set voice of Garfield.
According to an article in Premiere Magazine, much of Bill Murray's recording sessions were done aboard the boat he was occupying while filming The Life Aquatic with Steve Zissou (2004) in Italy. Apparently, Murray wasn't impressed with the recordings he did while on land.
The scene where Garfield gets flattened against the back windshield of Liz's truck is obviously a joke on the very popular Garfield plush toys that have suction cups on the paws so they can be stuck on windows. Garfield even takes the same pose as the plush toy, with the arms up diagonally.
The license plate on Liz's pick-up is "Pet Doc".
The song "New Dog State of Mind" that Garfield sings in the movie is based on the song "New York State of Mind" by Billy Joel.
The animal trainer wore blue gloves and pushed the real dog off the chair. Then the blue hands were digitally erased. The CG Garfield was animated and matchmoved with the real dog. Then Garfield was composted into the live-action plate, pushing the dog off the chair.
Cameo: [Jim Davis] Garfield's creator has a cameo as the train conductor who makes the "all aboard" announcement.